1. Field of Industrial Utilization of the Invention
The present invention relates to a coating method, which is used in the manufacture of printing plates such as gravure printing rolls, etc., for coating a roll that is to be made into a printing plate (called "printing roll" hereafter) with an extremely thin and uniform photosensitive film and for further coating a transparent protective film consisting of polyvinyl alcohol etc., after the photosensitive film has been coated.
2. Prior Art
Conventionally, coating of photosensitive film on a printing roll that is to be made into printing plates has been accomplished by an apparatus described in Japanese Patent Publication (Kokoku) No. 58-29471. Specifically, an annular dish which has a rubber seal formed around its interior circumference is fit over the printing roll (that is to be made into a printing plate). The printing roll is then set in a vertical position and held by a chucking means at its upper and lower ends. The annular dish is positioned at the upper end of the printing roll, and a photosensitive liquid is poured into the annular dish. The annular dish, connected to a raising/lowering device, is lowered, and when the annular dish has reached the lower end of the printing roll, lowering of the dish is stopped. Then, the photosensitive liquid remaining in the dish is recovered.
However, if coating is performed by the above-described apparatus and photosensitive liquid for laser exposure processes is used, the resulted film is so thick that a desirable laser exposure is not accomplished. Accordingly, a development of a coating technique which performs coating on a printing roll with a much thinner, uniform photosensitive film is demanded. Also, a coating technique wherein a transparent protective film consisting of polyvinyl alcohol, etc. is coated after the photosensitive film has been coated is demanded. Unlike conventional photosensitive liquids, the photosensitive liquids which are used in laser exposure processes require an overcoating with a transparent protective film consisting of polyvinyl alcohol, etc. In this case as well, an extremely thin and uniform coating is required.
In the effort to develop such a coating technique, an application method of an ink layer to printing rolls, a paste printing method used to paste two packaging films together, and a coating material printing method used for coated rolls in papermaking machines, were studied in order to determine whether or not such methods could be utilized to solve the problems. In all of these methods, however, since the rolls are not ideally parallel, uniform roll contact was not obtained for the entire length of the roll. Accordingly, since it is impossible to perform a coating operation so that the thickness of the coating film is uniform, fatal drawback such as those below. At the point where the contact with the printing roll (that is to be made into a printing plate) is released, the coating liquid is transferred to the printing roll, accumulating linearly too thickly.